1. Field of the Invention
The invention relates to a method for data communication along a pipeline and to a data transmission system for a pipeline.
2. Background of the Invention
In order to control and monitor pipelines, communication signals such as monitoring signals of pump stations, control commands or filling level indications have to be communicated to stations or central control stations arranged along the pipeline. In known systems, this data communication is effected either via telephone lines, by radio wave communication or via dedicated pilot cables laid along the pipeline. However, equipment and installations which are intended to be connected by the communication system are often situated at remote and/or not easily accessible locations. These regions are often not covered by the telephone network and, in some instances, also do not permit a pilot cable to be laid. Moreover, all three types of communication systems are associated with relatively high costs, because of the connection costs in the case of the telephone line, because of the additionally necessary installations in the case of radio wave communication and the pilot cable, and also because of the increased outlay on maintenance in the case of the pilot cable.
U.S. Pat. No. 5,785,842 discloses a data communication system for pipelines which is limited to the communication of data regarding corrosion protection. A monitoring device is connected to an active cathodic corrosion protection unit, and it conducts values from the latter via a satellite to a central control station.
Cathodic corrosion protection units of this type are disclosed in the prior art. They exploit the fact that pipelines which transport gaseous or liquid media are usually laid in a manner such that they are electrically insulated relative to the ground potential. This insulation can be attained by means of an insulation layer applied to the pipeline or its pipes, for example in the form of coats of paint, bitumenization or plasticization. For insulation purposes, the pipeline can also be embedded in sand and gravel or be laid on electrically insulated supports above the ground.
Two types of cathodic corrosion protection units are known. A first type, the passive corrosion protection unit, has sacrificial electrodes, which are produced from a suitable material and are laid in the ground. These sacrificial electrodes are connected to one another and, at grounding points spaced apart from one another, to the pipeline. On account of an electrochemical series, a potential which causes a current flow is produced. As a result, the sacrificial electrodes corrode, but not the material of the pipeline. In a second type, the active corrosion protection unit, a metallic pipeline is connected to an electrical energy source in order to raise the pipeline to an electrical protective potential. For this purpose, the corrosion protection unit has a rectifier fed by an AC voltage. The rectifier is connected to the pipeline by one pole and to ground by an opposite pole, as a result of which defined grounding points are present in this case as well. A long pipeline normally has a plurality of such active corrosion protection units arranged distributed over its length.
The object of the invention is to provide a method for data communication along a pipeline and also a data transmission system for a pipeline which enable cost-effective communication even in remote regions.
According to the invention, the pipeline itself is used as a data line by electrical signals being modulated onto a potential of the pipeline and being transmitted via the pipeline, which is electrically conductive at least in sections.
The electrical signals are preferably transmitted onto the pipeline at defined grounding points thereof, the electrical signals being modulated onto a DC protective signal applied to the pipeline in the case of active cathodic corrosion protection. By virtue of the decoupling of the DC protective signal, its average value and hence the corrosion protection remain unburdened by the electrical modulation signal.
In order at least approximately to prevent the modulation signal from flowing away to ground, a blocking element, for example a filter or an inductance, is preferably present.
In order to avoid interference to the best possible extent, it is possible, in particular for long transmission links, to use a frequency range of 1-100 kHz, preferably 4-10 kHz, for the modulation signals. The choice of a high frequency range, preferably above 1 kHz, means that ions of the pipeline do not have a high degree of mobility, so that no corrosion is caused. This allows the sign of the transmission signal or of the resulting instantaneous voltage to be momentarily changed.
It is advantageous that even existing pipelines can be retrofitted with the data transmission system according to the invention.